1. Field of the Invention
The present invention relates to a control unit having a memory in which a program consisting of command words is stored, corresponding to which either function signals are delivered, or, depending on the presence of a condition, a transfer to another command word present in the memory is undertaken. A condition multiplexer is addressed by the memory and receives condition signals and emits a load signal upon the presence of a condition indicated in the command word. An addressing unit is provided in which the actual addresses of the command word to be targeted in the memory resides and into which the transfer address of the next command word to be carried out is emitted from the memory upon the application of a load signal from the condition multiplexer.
2. Description of the Prior Art
Recently, microprogrammable control units, for example for peripheral devices, have been increasingly applied for the realization of complex control tasks. These control units offer a high flexibility in modification and expansion of their functions, since the essential portion of the control properties is realized by means of a microprogram and modifications can be achieved by the substitution of the microprogram. A disadvantage of microprogrammable control units is that operations, in contrast to pure hardware controls, can only run in temporal succession. This leads to significant limitations in the use of microprograms in very rapid control units.
At the present time, primarily microprocessors in MOS technology are being used for the realization of these microprogrammable control units. Conditioned by this technology and by the broadly fanned, command block provided for the wide area of application, command times of at least a number of microseconds result. In comparison thereto, bipolar microprocessors have significantly shorter command times, but require significantly more mounting space and power.
It appears, moreover, that, of the multitude of the commands of the microprocessor in control units for peripheral devices, usually only a small number of commands is required. These commands have the following two functions: interrogation of changeable states of the system to be controlled (conditions), modification (control) of the states of the system to be influenced, for example, on the basis of the interrogated conditions. By waiving the commands which are not required, a considerable simplification of the logical structure of the microprocessor results and, therefore, also a decrease of the required command times. The control unit then resulting can be built with TTL modules and programmable read-only memories (PROM). With such a control unit, command and cycle times of under 100 nanoseconds are achieved.
Such a control unit is known from the publication "ELEKTRONIK" 1975, Vol. 1, Pages 53-57, which is fully incorporated herein for its teaching of this structure and environment. The control unit comprises a memory (PROM), a condition multiplexer and a status counter (address unit). The status counter contains the respective actual address of the command word contained in the memory which is to be executed. A clock pulse is applied to the status counter, which always advances the counter by a unit when a command has been executed. Furthermore, however, the status counter can also be loaded with an address. The programs with the command words are contained in the memory. The memory has outputs for emitting function signals, by means of which the control signals, for example, in a peripheral device, is triggered. It also has further outputs for emitting addresses for the condition multiplexer, on the one hand, and for the status counter, on the other hand. In particular, when in the sequence of the program a command word occurs according to which a condition is to be interrogated, then the condition multiplexer is addressed, to which the individual conditions are supplied as condition signals. The condition multiplexer determines whether or not the condition is fulfilled. If, for example, the condition is fulfilled, it causes the status counter to emit the address for the next command to be executed out of the memory. The emission of the transfer address from the memory is carried out upon the application of the next clock pulse.
A further embodiment of such a control unit may be derived from the German Offenlegungsschrift No. 622,964, also incorporated herein by this reference. This control unit likewise contains a memory, a condition multiplexer and an address unit. A format of the control unit is illustrated according to which, upon the presence of specific commands, desired functions can be executed.
The known control units, however, have the disadvantage that a sequence of a program cannot be interrupted from the outside and that a call-in of a subprogram by means of a command is not possible.